1. Technical Field
The disclosed embodiments relate determining a bit rate and transmit power for energy-efficient packet transmission.
2. Background Information
Knowing the location of a non-AP station (for example, a cellular telephone handset having a WiFi capability), on the handset, as the user carries the handset throughout an area served by a plurality of APs (Access Points) is useful and has multiple applications. One way to determine the location of a handset involves triangulation between APs. Distances from the handset to a number of APs are determined, and these distances and other information about the locations of the APs are then used to determine the location of the handset. A signal-strength-based distance-determining method may be used. In a WiFi IEEE 802.11 network, a handset can transmit to an AP using different bit rate settings and different transmission power settings. The combination of the bit rate setting and the transmission power setting determines the overall energy required to transmit a bit of a transmission. The AP periodically transmits transmissions called beacons. The transmit power with which the AP transmits a beacon could be estimated over time by the handset. The handset that receives a beacon is able to make an assessment of signal strength (RSSI) as the beacon is received at the handset. From the known transmit power with which the beacon was sent, and from the measured RSSI of the beacon as received, the handset can make an assessment of channel loss. Channel loss from the AP to the handset and from the handset to AP is generally symmetric. There is a relationship between channel loss and distance, so from the determined channel loss information the handset can make an approximation of the distance between the handset and the AP. Unfortunately, the environment and obstructions between a transmitter and a receiver can affect channel loss. For the same distance between a handset and an AP there may be different signal strengths as detected at the receiver depending on different environmental conditions and obstructions that might exist between the AP and the handset. Consequently, the determination of distance can have substantial error and is somewhat unreliable and environmentally dependent.